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Weekly Dry Season Situation Report for the Mekong River Basin - Prepared at: 12/11/2019, covering the week from 1st to 11th November 2019

Situation Report
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Weather Patterns, General Behaviors of the Mekong River and Outlook Situation

General weather patterns:

The weather of this week was brought moderate rainfall in the LMB. Rainfall in this week was considered below average, covered in the Luang Prabang and Thakhet areas, varied from 0.1 mm to 50 mm. The below- average rainfalls were found from Chiang Sean to Nong Khai, while from Nakhon Phanom and Kratie were considered above average condition, during this week. Rainfall distribution in October 2019 is depicted in Figure 1.

General behavior of the Mekong River:

This week from 1 st to 11th November 2019, water levels from Chiang Sean were increased slightly due to the inflow from China, varied from 0.03 m to 0.33 m but still stay below its Minimum level. However, water level trend at Luang Prabang and Chiang Khan are likely impacted by hydropower dam at Xayaburi, in which they fluctuated, varied from 0.02 m to 0.76 m. Water levels at stations at the middle part of LMB from Vientiane to Pakse decreased and stay below their minimum levels. Follow the same trend of water levels from upstream, stations at Stung Treng, Kratei, Chaktomuk on the Bassac, Phnom Penh Port and Neak Luong were also below minimum levels. For the 2 tidal stations at Tan Chau and Chau Doc, water levels decreased since middle of October 2019, and stayed below their LTAs.

For stations from Chiang Saen and Luang Prabang

Water levels from 1 st to 11th November 2019 at Chiang Sean station were slightly increased, due to the decreased flow from Jinghong on 23 Oct 2019 (see its hydrograph in Annex A). At this station water levels increased from 0.03 m to 0.33 m. However, at Luang Prabang station, water levels were fluctuated and stay below its LTA. Water levels at this station sometime increased rapidly in 0.66 m, due to the reservoir operation of upstream and downstream at Xayaburi. It was observed that the Luang Prabang stations is likely nominated by hydro power dam operation upstream (tributaries) and downstream (Xayaburi) in which water levels always fluctuated above their LTAs, during the impounding reservoir at Xayaburi from end of October 2018 to May 2019.

For stations from Chiang Khan, Vientiane-Nong Khai and Paksane

Water levels from 1 st to 11th November 2019 at Chiang Khan station were likely also nominated by upstream hydropower dam of Xayaburi, which was noted that water levels fluctuated from -0.76 to 0.44 m (see its hydrograph in Annex A). The current observed water levels at Vientiane/Nong Khai and Paksane stations are lower than their historical minimum levels and drought year of 1992.

For stations from Nakhon Phanom to Pakse

Water levels from 1 st to 11th November 2019 at Khong Chiam to Pakse stations were also slightly increased, followed the inflow from upstream. The water levels decreased from 0.01 m to 0.08 m. The current water levels at these stations below their minimum historical levels.

For stations from Stung Treng to Kompong Cham/ Phnom Penh to Koh Khel/Neak Luong

Water levels from 1 st to 11th November 2019 at Stung Treng, Kratie and Kompong Cham were continued to decrease, varied from 0.02 m to 0.27 m. The current water levels at Stung Trend, Kratie, Kompong Cham, Chaktomuk Koh Khel, Phnom Penh Port, Neak Luong and Prekdam on the Tonle Sap were below their historical minimum levels, while at (1980-2018).

Tan Chau and Chau Doc

Water levels from 1 st to 11th November 2019 at the 2 tidal stations at Tan Chau and Chau Doc were slightly increased, follows the tidal effect from the sea. Water levels of these stations were stay below their LTAs.

According to the Japan Meteorological Agency (JMA), Sea surface temperature (SST) variability in the tropics Neutral, which has no major impact.

Discussion and Conclusion

From 1 st to 11th November 2019, the trend of water levels at Chiang Sean were slightly increased due to the inflows from Jinghong from 03th Nov 2019. Water level at Chiang Sean is relied from inflow from Jinghong Hydropower Station on Lancang and its catchment rainfall. The impact could obviously see the gradually decreasing water level to downstream to Vientiane/Nong Khai. Based on a hydrological phenomenon, the inflow contribution of water from the upstream of LancangMekong in China to the Mekong mainstream is about 16% in total during the Dry season from Nov to May, while 24% in the Wet season (Adamson. 2010). The whole inflow of water into the lower Mekong basin is influenced more by inflow from tributaries and the direct rainfall catchment. The low inflows from upstream and less rainfall in catchments, resulting water levels from Paksane to Pakse are drastically dropped below their minimum levels. However, from Stung Treng, Kratie and Kompong Cham stations water levels are followed the same trend from upstream which stay below their minimum levels.

Due to the low flow of the Mekong, the upcoming Dry Season can be possible of facing some problem, related to the shortage of drinking water and agricultural production, fishery production, ecological systems, biodiversity, bank erosion, salinity intrusion in the Mekong Delta and waterway transport because not enough water for fish spawning and also aquatic lives etc. The reduced water flow could also affect to the expanding unsaturated soil which cause bank erosion and salinity intrusion from the sea.